In using aluminum plates as supports for printing plates, they are usually subjected to a treatment for roughening their surfaces in order to ensure good intimate adhesion between the aluminum plate and a light-sensitive film to be provided thereon and improve water retention in non-image areas. This surface roughening treatment is called graining, and includes mechanical graining such as ball graining, sand blast graining or brush graining, electrochemical graining which is also called electrolytic polishing, and chemical etching which is called chemical graining. These conventional graining processes possess advantages and disadvantages. In general, problems with the mechanical graining process include scuff marks, stains and residue of an abrasive used. The electrochemical graining process makes it possible to change the depth of the graining as well as the form of grains by controlling the quantity of electricity. However, it requires a large quantity of electricity and a long time to create grains suited for printing plates which leads to high production costs.
On the other hand, the chemical graining process grains aluminum or aluminum alloy by a chemical etching reaction using an acid or an alkali etchant, and, hence, it is simple and suited for continuously treating aluminum or aluminum alloy strips, and is particularly advantageous for industrially producing plates having been treated on both sides.
However, it has so far been difficult to produce high quality printing plates using commercially available aluminum or aluminum alloy. Conventional chemical etching processes described in, for example, U.S. Pat. Nos. 2,344,510 and 2,714,066, have difficulty in forming a surface having enough surface roughness and uniform pit pattern (wherein etching pits have a uniform diameter and a uniform depth) to give sufficient printing durability and staining resistance required for printing plates.
According to the experiments conducted by the inventors, chemical etching of commercially available aluminum and aluminum alloy (JIS 1050, 1100 and 3003) using various etchants has been found to involve the following problems. (1) It is difficult to provide a practical surface roughness of 0.3 to 1.2 .mu.Ra (centerline average roughness) which is suitable as a printing plate and, even when such roughness is attained, the reaction rate is so slow that the process requires a long time. (2) The etchant contains ingredients harmful for workers, thus causing a problem in view of working atmosphere. (3) The etching cost is too high to be practical.